1. Field of the Invention
The present invention relates to a sampling phase selection method for a data stream and, in particular, to a sampling phase selection method for a data stream having a variable data rate in a fixed time period.
2. Description of the Related Art
Most electronic devices may be driven by a clock signal during operation. With rapid developments in technology, a clock signal may operate at an operating frequency in the order of gigahertz (GHz) or higher. A clock signal may have a stable oscillating frequency, which is generated by an oscillator.
However, a desirable oscillator has a relatively high Q value, which is the allowed energy of the clock signal concentrated on a narrow baseband and the harmonic wave of the baseband. When the energy is concentrated on a high frequency harmonic wave, the radiation produced by Electro-Magnetic Interference (EMI) may be greater than regulation standards made by FCC, JEITA and IEC.
For reducing EMI, a Spread Spectrum (SS) is utilized to modulate the clock signal. The frequency of the SS modulated clock signal may not be fixed at a certain frequency but may be varied within a given frequency range. Therefore, the clock signal may have a lower energy distribution or lower frequency range to reduce the effect of EMI.
FIG. 1A shows a clock signal CLK_ref which is not modulated by SS. As shown in FIG. 1A, the clock signal CLK_ref has a fixed frequency fc.
FIG. 1B shows a frequency spectrum of the clock signal CLK_ref. As shown in FIG. 1B, the energy at frequency fc is above a given threshold energy P0. The given threshold energy P0 is the energy for causing EMI.
FIG. 1C is a diagram showing the clock signal CLK_ref over time.
FIG. 2A shows a clock signal CLK_SS which is modulated by SS.
FIG. 2B shows a frequency spectrum of the clock signal CLK_SS. As shown in FIG. 2B, the energy of the clock signal CLK_SS has been spread to frequency ranges f1 and f2. Therefore, energy at frequency fc is under a given threshold energy P0.
FIG. 2C is a diagram showing the clock signal CLK_SS over time. Referring to FIG. 2C, the output frequency of the clock signal CLK_SS may periodically work between a range of frequency f1 and f2.
Moreover, a high speed data stream may be modulated by SS and transmitted in order to reduce EMI. When the modulated data stream is transmitted, data jitter and phase skew between the modulated clock signal and the data stream may affect the sampling section of significant bits.
If a conventional oversampling is performed to sample bits of data of the modulated data stream, a certain sampling clock phase may only sample bits of data of a certain frequency band. Moreover, when a high-speed data stream is transmitted via a noise channel, the high-speed data stream may be interfered by a noise signal having a fixed time section.
If a conventional oversampling is performed on the high-speed data stream, the noise signal may be continuously sampled by a certain sample phase. Therefore, the current disclosure provides a method for selecting a sampling phase of a data stream.